Monotremes are a unique group of animals made up of the echidna and the platypus. These animals lay eggs from which their young emerge, like a reptile, but are 'warm-blooded' and suckle their young, like a mammal.

Lead author Dr Matt Phillips, of the Australian National University (ANU) in Canberra, says the echidna has evolved from the semi-aquatic platypus-like form to the modern terrestrial spiny anteater in a relatively short time.

The research was prompted by a paper last year that suggested two Australian fossils from the early Cretaceous - the time of the dinosaurs - were close relatives of the platypus.

The linking of this fossil to the platypus lineage would have meant that platypuses and echidnas diverged more than 112.5 million years ago, reinforcing the notion of monotremes as living fossils.

DNA rewind

Phillips and his colleagues challenged this view by using molecular dating of DNA sequences from the modern-day platypus and echidna to show the two species shared the same ancestor about 30 million years ago.

Under molecular dating the researchers look at how different modern DNA sequences are and estimate when those DNA sequences would have been similar.

"We know the common ancestor would have been more platypus than echidna," says Phillips, "[because] fossils dating back to 60 million years reveal a monotreme that was slightly larger than the modern-day platypus, but very anatomically similar."

He says the result also explains the lack of fossil record for the echidna before 13 million years ago.

"There is no fossil record [dating before 13 million years] because they simply hadn't evolved yet," he says.

Dominant species

Before the arrival of marsupials about 74 million years ago, monotremes were the dominant mammal on the Australian continent, Phillips, of the ANU's Research School of Biology, says.

But only the lineage leading to platypuses and echidnas survived the marsupial invasion, he says.

He says monotremes would have only been able to survive by occupying ecological niches where they would have faced the least competition.

Phillips and his colleagues, Thomas Bennett, also from ANU, and Dr Michael Lee from the South Australian Museum, suggest the echidna and platypus found territory in which marsupials were restricted by their reproductive system.

"After birth, marsupial young begin an extended period of fixation onto a nipple. This both compromises the ability of the mother to forage in aquatic environments and provides an early development al constraint on the evolution of an anteater-style 'beak'," they write.

"The echidna certainly isn't a living fossil," says Phillips. "It is something that has evolved a considerable amount [and] captured a new ecological space in potential competition against marsupials that were there well before the echidna evolved."

Phillips says there are a number of anatomical features that also support their finding.

Although it is no longer amphibious, the echidna can "swim very well and uses its bill like a snorkel", says Phillips.

Its hind feet are also reversed, and it retains the ankle spurs that are venomous in platypuses, but non-functional in the echidna.